Dopamine, a major catecholamine neurotransmitter, is involved in the regulation of a variety of functions which include emotion, cognition, motor functions, and positive reinforcement. The biological activities of dopamine are mediated through G protein-coupled receptors (GPCRs) and in human, five different dopamine receptors D1-D5 have been identified, where the D2-like receptors (D2, D3 and D4) couple to the G-protein GαI. The D3 dopamine receptor is most highly expressed in the nucleus accumbens and is proposed to modulate the mesolimbic pathway consisting of neuronal projections from the ventral tegmental area, hippocampus and amygdala to the nucleus accumbens, which projects to the prefrontal and cingulate cortices as well as various thalamic nuclei. The limbic circuit is thought to be important for emotional behavior and thus D3 receptor antagonists are proposed to modulate psychotic symptoms such as hallucinations, delusions and thought disorder (Joyce J. N., Millan M. J., Drug Discovery Today (2005) 10:917-925). In addition, it has been reported that drug naive schizophrenic patients show altered levels of D3 receptor expression (Gurevich E. V. et al., Arch. Gen. Psychiatry (1997) 54, 225-232) and dopamine release (Laruelle M., Presentation at Institut de Recherches Internationales Servier Workshop on Schizophrenia: Pathological Bases and Mechanisms of Antipsychotic Action, Chicago, Ill., 2000), indicating that a disturbed homeostasis of dopamine plays an important role in the etiology of schizophrenic symptoms.
The neurotransmitter serotonin (5-Hydroxytryptamine; 5-HT) is implicated in several psychiatric conditions including schizophrenia (Kandel E. R. et al. (eds.), Principles of Neural Science (2000) 3rd edition, Appleton & Lange, Norwalk, Conn.). The involvement of serotonin in psychotic disorders is suggested by multiple studies which include treatment in humans with the psychotropic drug Lysergic acid (LSD; a serotonin agonist) which can induce schizophrenia-like symptoms such as hallucinations (Leikin J. B. et al., Med. Toxicol. Adverse Drug Exp. (1989) 4:324-350). Furthermore, altered brain distribution of serotonin receptors as well as an altered serotonergic tone, have been detected in schizophrenic patients (Harrison P. J., Br. J. Psychiatry Suppl. (1999) 38:12-22).
In mammals, serotonin exerts its biological activities through a family of 14 5-HT GPCRs. The 5-HT2A receptor is most prominently expressed in the prefrontal cortex and at lower levels in the basal ganglia and the hippocampus in human brain, and is coupled predominantly to the G-protein Gαq. Genetic linkage studies of a 5-HT2A polymorph to schizophrenia (Spurlock G. et al., Mol. Psychiatry. (1998) 3:42-49), as well as responsiveness to antipsychotic drugs (Arranz, M. J. et al., Lancet (2000) 355:1615-1616), further suggest a role for the 5-HT2A receptor both in the treatment and pathology of psychosis. In addition, dopaminergic neurotransmission appears to be under the afferent regulation of the 5-HT2A receptor (Porras G. et al., Neuropsychopharmacolo-gy (2002) 26:311-324). Overall 5-HT2A receptor antagonists are proposed to be suitable for the treatment of disorders associated with dysfunctional dopaminergic systems. Moreover, 5-HT2A receptor antagonism has been recognized as beneficial for the treatment of psychosis (de Angelis L., Curr. Opin. Investig. Drugs (2002) 3:106-112).
The D3 and 5-HT2A receptors besides the mentioned psychotic disorders are further reported to be linked to other psychoses including paranoia and delusions (Reavill C. et al., JPET (2000) 294:1154-1165; Harrison P. J., Br. J. Psychiatry Suppl. (1999) 38:12-22), to drug dependency, abuse and withdrawal (Vorel S. R. et al., J. Neurosci. (2002) 22:9595-9603; Campos A. C. et al., Soc. Neurosci. Abstr., (2003) 322:8; Ashby C. R. et al., Synapse (2003) 48:154-156), attention deficit hyperactivity disorders (ADHD) (Retz W. et al., J. Neural. Transm. (2003) 110:531-572; Levitan R. D. et al., J. Affective Disorder (2002) 71:229-233), as well as to anxiety and depression (Reavill C. et al., JPET (2000) 294:1154-1165; Drescher K. et al. Am. Soc. Neurosci. (2002) 894:6).
Currently used medications to treat schizophrenia, bipolar mania and other psychoses, include both typical (D2/D3 preferring) or the more recent atypicals, which exhibit polypharma-cology interacting at multiple receptors (e.g., D1, D2, D3, D4, 5-HT1A, 5-HT2A, 5-HT2C, H1, M1, M2, M4, etc.)(Roth B. L. et al., Nat. Rev. Drug Discov. (2004) 3:353-359). These antipsychotics, although relatively successful (some patients exhibit treatment resistance) at treating the positive symptoms of schizophrenia, are less effective at treating negative symptoms, cognitive deficits, and associated depression and anxiety, all of which lead to reduced patient quality of life and socioeconomic problems. Furthermore, patient compliance is compromised by prevalent side effects such as weight gain, extrapyramidal symptoms (EPS), and cardiovascular effects (Lieberman J. A. et al., N. Engl. J. Med. (2005) 353:1209-1223).
Antipsychotic drug treatment has frequently been complicated by serious side effects of widespread D2 antagonism, notably an extrapyramidal or parkinsonian syndrome caused by antagonism of the dopaminergic projection from substantia nigra to corpus striatum. D2 receptor blockade induces catalepsy and has been associated with negative effects against cognition. Also preferential blockade of D3 vs. D2 receptors, preserves and/or enhances cognitive function, and increases frontocortical cholinergic transmission. (Joyce J. N., Millan M. J., Drug Discovery Today (2005) 10:917-925, Moore N. A. et al., European Journal of Pharmacology (1993) 237:1-7; Barth V. N., Typical and atypical antipsychotics: Relationships between rat in vivo dopamine D(2) receptor occupancy assessed using LC/MS and changes in neurochemistry and catalepsy. Dissertation Indiana University (2006); Millan M. J. et al., Fr. Journal of Pharmacology and Experimental Therapeutics (2008) 324:1212-1226; Wiecki T. V. et al., Psychopharmacology (2009) 204:265-277).
The typical antipsychotic agents on the market today display D2 antagonism, and most have extrapyramidal side effects (EPS) such as pseudoparkinsonism and tardive dyskinesia (Howard H. R., Seeger T. F., Annual Reports in Medicinal Chemistry (1993) 28:39). It has been shown by selective binding experiments that D2 receptors are more concentrated in the striatal regions of the brain, which are responsible for locomotor control than in the limbic regions which are responsible for thought processes. D3 receptors are more concentrated in the limbic than in the striatal regions. It is therefore believed that selective D3 ligands may relieve symptoms of schizophrenia without causing the EPS associated with blockade of D2 receptors (Gackenheimer S. L. et al., J. Pharmacol. Exp. Ther. (1995) 274:1558, Belliotti T. R., Bioorg. Med. Chem. Lett. (1997) 7:2403).